Your theme imposes rather generic styles on li elements in the content areas under a parent class of ‘shortcodes’ This is conflicting with the BP object nav li elements forcing them to 100% width.

In your themes stylesheet you’ll need to try adding something like:

<code>

#buddypress div.item-list-tabs ul li {
width: auto;

}

</code>

Other than that it’s a case of using that top level id #buddypress to further style BP elements in what ever stylesheet is applicable using tools such as firebug to identify the rulesets and their selectors that BP has already set.